The present invention concerns ignition glow-plugs in which the basic matrix phase of both the conducting and insulating elements is made of a same ceramic, electrical conductivity of the conducting elements being provided by particles of one or more comminuted conductive materials dispersed in said matrix phase. The ignition glow-plugs of this invention are usable as fast response ignition plugs in high-compression thermal engines, e.g. Diesel engines. The invention also deals with a method for fabricating ceramic ignition glow-plugs.
To start high-compression engines under cold conditions, one uses electrical ignition glow-plugs which must reach the operational temperature (1000.degree. C. or more) before the starter motor is switched on. Now, the time required to preheat glow-plugs may last, depending on the outside temperature, from a few seconds to several tens of seconds because the heating element of the plug has a substantial degree of thermal inertia; hence one has sought to reduce the delay as much as possible by using very large heating currents as well as automated systems for controlling this current when the desired temperature is attained, thereby avoiding premature deterioration of the plug. When a glow-plug normally operates under the foregoing conditions, it is subject to high stress and thermal shocks which threaten to prematurely end its operating life.
Moreover, when the motor is in normal operation, the fuel combustion effects in the cylinders followed by the rapid cooling due to the outflow of exhaust gases will also contribute, together with the heat developed by the glow-plug, to generate thermal oscillations which may result in cracking and premature failure of the plug components, especially if the thermal expansion factors of the insulating and conducting components are markedly different from one another.
These problems are mentioned in documents U.S. Pat. No. 4,931,619 and U.S. Pat. No. 4,742,209 (JIDOSHA-HITACHI) in which it is proposed to use a ceramic matrix for making both the electroconducting and insulating portions of the glow-plug. This concept is validated by using an electrically conductive ceramic for making the heater portion of the plug, whereas the insulating portion is made of insulative ceramic. In order to achieve this object practically, the foregoing documents particularly recommend a SiALON type ceramic. This ceramic is normally insulative without additives; it becomes conductive with the addition of a proportion of titanium nitride. In an embodiment of this achievement, SiALON and titanium nitride are sintered together by using, for thermal compaction, sintering aids such as Y.sub.2 O.sub.3, AlN and Al.sub.2 O.sub.3.
Document U.S. Pat. No. 4,742,209 further proposes other ceramic types convenient to manufacture glow-plugs, inter alia ceramics that can resist temperatures of 1200.degree. C. These ceramics include conductive types like carbides, borides and nitrides, particularly SiC, and insulative types such as Si.sub.3 N.sub.4, AlN and Al.sub.2 O.sub.3.
Also document U.S. Pat. No. 4,486,651 (NIPPON SOKEN) discloses a heating body comprising a conductive mixture of MoSi.sub.2 and Si.sub.3 N.sub.4 bound to an insulating substrate of Si.sub.3 N.sub.4 or Al.sub.2 O.sub.3. In an embodiment, the heating body is in the form of an ignition glow-plug.
Document EP-A-335.382 (NIPPON DENSO) discloses ignition glow-plugs of which an embodiment comprises a Si.sub.3 N.sub.4 insulator substrate and a heating component consisting of an admixture of Si.sub.3 N.sub.4 in 10 .mu.m particles and Mo.sub.5 Si.sub.3 C in 1 .mu.m particles. In a particular variant of this embodiment, the insulator substrate also contains a proportion of particulate conductive MoSi.sub.2, but the particle size of the Si.sub.3 N.sub.4 (1 .mu.m) is much smaller than that of the Si.sub.3 N.sub.4 particles (10 .mu.m) of the conductor element; hence the many MoSi.sub.2 particles do not touch one another and the material is not electrically conductive. Notwithstanding, having the two materials, the insulative and the electrically conductive ones, in both the conducting and insulating components of the plug (although the proportion in each are different) will cause the thermal expansion factors in both components to be much alike, which strongly reduces internal stresses with temperature changes.
Also U.S. Pat. No. 4,634,837 (NIPPON SOKEN) discloses sintered ceramic glow-plugs. In an embodiment, the heating component comprises a sintered mixture of Si.sub.3 N.sub.4 powder and MoSi.sub.2 powder the particle size of the former being smaller than the particle size of the latter. The insulating component comprises Si.sub.3 N.sub.4 and Al.sub.2 O.sub.3 powders in sintered admixture. It appears clearly from the teaching of this document that for a given fixed weight ratio of conductive (MoSi.sub.2) and insulative particles (Si.sub.2 N.sub.4) in the conducting element of the glow-plug, the effective conductivity will increase in function to the magnitude of the ratio of particle sizes of the Si.sub.3 N.sub.4 and MoSi.sub.2.
The main advantage of the glow-plugs of the aforediscussed prior art is resistance to thermal shock due to admittedly small differences in the thermal expansion factors of the ceramic matrices involved in making the conducting and insulating elements. As mentioned previously, this small difference is due to using for instance a same ceramic base matrix for both the conducting and insulating components, the conducting component (the heating body of the plug) simply comprising, in admixture with the ceramic base, a conductive ceramic in sufficient quantity to assure electrical conductivity and consecutive electrical heating properties by the Joule effect.